1. Field of the Invention
The present invention relates to an image-forming apparatus for use in monochromic/full-color copying machines, printers, fax machines, multifunctional processing machines thereof etc.
2. Description of the Related Art
In a full-color image-forming apparatus in the intermediate transfer process, a color image is formed, for example, by once transferring toner images different in color that are developed on multiple photosensitive members onto an intermediate transfer member to be superimposed and transferring the superposed toner images all together onto an image receiving medium such as paper. The transferring process from photosensitive member to intermediate transfer member is called primary transfer, while that from intermediate transfer member to image receiving medium, secondary transfer. In these image transfer processes, the toner is driven to transfer in an electric field formed by applying bias voltages, for example, to the transfer rollers. For example, primary transfer voltage is applied to the primary-transfer roller in the primary transfer process, while secondary transfer voltage, to the secondary-transfer roller in the secondary transfer process, for transfer of the toner.
In such an image-forming apparatus, the surface of the intermediate transfer member is electrified by application of the secondary transfer voltage in the secondary transfer process. However, electrification of the intermediate transfer member surface is not uniform in the secondary transfer process. For example when paper in smaller size is fed, the intermediate transfer member becomes in contact directly with the secondary-transfer roller in the peripheral regions and indirectly via the paper in the central region, and thus, the surface of the intermediate transfer member surface is charged unevenly. Even in the region where the intermediate transfer member becomes in contact via paper with the secondary-transfer roller, an area in image region where the toner is present in a relatively greater amount, for example, is resistant to electrification of the intermediate transfer member surface, while an area such as white area where the toner is present in a relatively smaller amount is electrified more readily. Thus when the intermediate transfer member surface is electrostatically charged unevenly, the electrostatic charge distribution on the surface forms a latent image, causing formation of residual images thereof (irregularity in density) in next images.
Accordingly proposed is a method of preventing such transfer irregularity, by reducing the residual potential of the intermediate transfer member to ½ or less before the subsequent transfer (Japanese Unexamined Patent Publication No. 2004-157,265). However, it is not possible to prevent the residual image sufficiently even by such a method.
On the other hand, an intermediate transfer member having a surface layer, which is made of a material different from that for the substrate, formed on the outermost surface have been used recently for improvement in image quality. The surface layer, which is formed for improvement of roughness, hardness and surface properties such as toner release characteristics of the intermediate transfer member surface, often has electrical properties different from those of the substrate. The intermediate transfer member having such a surface layer may become significantly law in static elimination, depending on the lamination conditions of the surface layer, and thus, may generate more remarkably residual toner images described above.